FIG. 1 shows a cellular communication system with a serving node 10 (depending on the system, it can be called a base station, a Node B, an evolved Node B (eNodeB or “eNB), etc.) that serves a user equipment (UE) 12 located within the serving node's geographical area of service, called a cell 14. Communication is bidirectional between the eNB 10 and the UE 12. Communications from the eNB 10 to the UE 12 are referred to as taking place in a downlink direction, whereas communications from the UE 12 to the eNB 10 are referred to as taking place in an uplink direction.
In E-UTRAN, Orthogonal Frequency Division Multiple Access (OFDMA) technology is used in the downlink, and single carrier frequency division multiple access (SC-FDMA) in the uplink. In both the uplink and downlink, the data transmission is split into several sub-streams, where each sub-stream is modulated on a separate sub-carrier. Hence, in OFDMA based systems, the available bandwidth is sub-divided into several resource blocks (RB). A resource block is defined in both time and frequency. According to the current assumptions, a resource block size is 180 KHz and 0.5 ms in frequency and time domains, respectively. The overall uplink and downlink transmission bandwidth can be as large as 20 MHz. Carrier aggregation in LTE enables the UE to substantially enhance the data rate by simultaneously receiving and transmitting data over more than one component carrier.
The UE must be in compliance with relevant national and international standards and regulations regarding human exposure to radiofrequency (RF) electromagnetic fields (EMF). The exposure limits specified in these standards have been adopted from guidelines provided by the International Commission on Non-Ionizing Radiation Protection (ICNIRP, 1998) or from the C95.1 standard developed by the Institute of Electrical and Electronics Engineers (IEEE C95.1, 1999). The limits in these recommendations are similar and they have been based on the same scientific data. The ICNIRP guidelines, which are the most widely used recommendations, have been endorsed by the World Health Organization (WHO).
The science-based RF exposure limits specified in these guidelines have been set with substantial safety margins. They provide protection from all established health effects from short-term and long-term exposure to RF fields, and the safety of children and other segments of the population have been taken into account.
Specific Absorption Rate (SAR) is the quantity used to measure the RF exposure to RF EMF transmitted by the UE. SAR is a measure of the maximum energy absorbed by a unit of mass of exposed tissue, over a given time or more simply the power absorbed per unit mass. The ICNIRP SAR limit applicable for mobile phones and other UE used closed to the body is 2 W/kg averaged over 10 gram of tissue. This limit is used within the EU and most countries worldwide. The US Federal Communications Commission (FCC) has adopted the IEEE 1999 SAR limit of 1.6 W/kg averaged over 1 gram of tissue. This limit is also used in a few other countries. The SAR limit in Europe and in some other countries is 2 W/kg, which is set by the European regulators. In other regions of the world, the SAR requirements may be different.
Although SAR requirements are set by the regulators in different countries, they do not exist in the 3GPP specification. For example, no SAR requirements are specified for the GSM, HSPA, or LTE UEs. Due to lack of standardized/harmonized SAR requirements, different UEs may exhibit different SAR levels.
A UE should comply with the SAR limit issued by the individual government of the country/region/province. The UE SAR is measured with a model that assumes a phone is close to the ear. Hence the maximum UE output power is limited by the SAR limit. In the 3GPP HSPA specification 3GPP TS 25.101, “User Equipment (UE) radio transmission and reception (FDD),” only UE power class 3 and power class 4 are specified with the maximum output power for a UE of power class 3 being 24 dBm and for a UE of power class 4 being 21 dBm. In the 3GPP LTE specification, 3GPP TS 36.101, “Evolved Universal Terrestrial Radio Access (E-UTRA) and Evolved Universal Terrestrial Radio Access (E-UTRAN); User Equipment (UE) radio transmission and reception,” only UE power class 3 is specified with the maximum output power of 23 dBm.
There are multiple factors that impact the experienced SAR from a UE for an end user. Some factors are listed below as examples. One is a distance between the UE and the human body. According to the characteristic of the radio transmission, the electromagnetic wave strength decreases dramatically (e.g., several tens of dBs) when the phone moves from close to the ear (e.g., 1 cm) to a small distance (e.g., half or 1 wave-length). For a wireless communication system operating at around 2 GHz frequencies, the wavelength is around 15 cm. Example 1: For a UE used in machine to machine communication, the SAR experienced by the end user is far lower than SAR limit because the UE is far enough from the end user during operation. Example 2: When a user uses the UE (could be a cell phone or a dongle) as a wireless access adapter for a laptop or desktop to access the internet, the UE is located near the laptop and thus far enough from the end users. In this case, the SAR experienced by the end user is also far lower than the SAR limit imposed on the UE.
A second example factor is a distance between UE and base station. With a larger distance from the UE to the connected base station, a larger UE transmit power is needed to meet a required Quality of Service (QoS), which increases the SAR experienced by the end users. A third example factor is the surroundings and radio environment of the communication area. The obstacles between the UE and the base station can increase the electromagnetic wave propagation loss. In order to meet the required QoS, a larger transmit power is needed and consequently causes larger SAR experienced by the end user.
In addition to the SAR, the UE also has to fulfill a certain set of out-of-band (OOB) emission requirements. Some of these are set by regulatory bodies, e.g., ITU-R, FCC, ARIB, ETSI, etc. These out of band emission requirements are also referred to as regulatory radio requirements. The objective of OOB requirements is to limit the interference caused by the transmitter (UE or broadcasting equipment (BE)) outside its operating bandwidth to the adjacent carriers. Unlike SAR, the OOB requirements are well specified in 3GPP specifications. For a UTRA UE, they are specified in TS 25.101. For an E-UTRA UE, they are specified in TS 36.101. The OOB requirements typically comprise: adjacent channel leakage ratio (ACLR), spectrum emission mask (SEM), and spurious emissions, the specific definition of which can vary from one system to another. Furthermore, the OOB emission requirements have to be fulfilled on a time slot basis in WCDMA and on a sub-frame basis in E-UTRA.
The efficiency of a UE's power amplifier (PA) is an important factor in conserving the UE battery power. Therefore, an efficient PA will be typically designed for certain typical operating points or configurations, e.g., modulation type, number of active reference blocks (in the case of E-UTRA), number of physical channels/channelization codes/spreading factors (in the case of UTRA, which is based on CDMA technology). However, the UE may have to transmit using any combination of modulation, resource blocks, etc. Therefore, in some uplink (UL) transmission scenarios, the UE power amplifier may not be able to operate in the linear zone, thereby causing OOB band emissions due to harmonics. To ensure that UE fulfills OOB requirements for all allowed UL transmission configurations, (e.g., may include factors such as modulation type, number of resource blocks, etc.), the UE can reduce its maximum UL transmission power in some scenarios when it reaches its maximum power. This is called maximum power reduction or UE power back-off. For instance, a UE with nominal maximum output/transmit power of 24 dBm power class may reduce its maximum power from 24 dBm to 23 or 22 dBm depending upon the UL transmission configuration.
The maximum power reduction (MPR) values for different UL transmission configurations are generally well specified in the standard. The UE uses these values to apply MPR when the conditions for the corresponding configurations are fulfilled. These MPR values are regarded as static in a sense that they are independent of resource block allocation and other deployment aspects.
Additional maximum power reduction (A-MPR) may be needed to meet additional OOB requirements. In E-UTRA, an additional MPR (A-MPR) is also being specified on top of the normal MPR to meet additional OOB emission requirements, which cannot be met by only MPR. The difference between A-MPR and MPR is that the former is not fully static. Instead, A-MPR can vary between different cells, operating frequency bands, and between cells belonging to different location areas.
The A-MPR includes all the remaining power reduction (on top of the normal MPR) needed to account for factors such as bandwidth, frequency band, and resource block allocation to satisfy additional requirements such as requirements set by regional regulatory bodies (FCC, ARIB etc). Table 6.2.4-1, “Additional Maximum Power Reduction (AMPR),” in TS 36.101 illustrates how A-MPR requirements are currently defined for E-UTRA UE.
One possible approach is to specify a fixed, predefined SAR value which the UE should meet regardless of the scenario. But the SAR requirement may depend upon the scenario, e.g., country, region, regulator, standard, etc. In order to meet the fixed SAR requirement, the UE may have to reduce its maximum output power. In some scenarios, a fixed SAR value may result in excessive UE power backoff or in smaller than required reduction in UE output power. Furthermore, future SAR requirements may change, i.e., may become more stringent or less stringent. So there is a need for UEs to be adaptable to different and changing SAR requirements.